A continuously variable transmission system for a vehicle for example comprises a hydraulic transmission such as a torque converter or fluid coupling and a continuously variable transmission (referred to hereafter as CVT). The hydraulic transmission transmits an engine rotation output to CVT, and CVT changes the rotation speed of the engine output in a stepless mode and transmits it to a vehicle drive shaft.
CVT is disclosed for example in Tokkai Hei 3-121358 published in 1991, and Tokkai Sho 59-217047 published in 1984 by the Japanese Patent Office.
Such a CVT has an arbitrary gear ratio, a rotational force being communicated via for example a V-belt looped around a drive pulley and a driven pulley. The speed change ratio can be continuously varied by increasing the width of one pulley and decreasing the width of the other, and oil pressure supplied through a gear control valve acts on each pulley so as to change its width. The speed change ratio is therefore varied by increasing and decreasing the opening of the speed change ratio control valve.
In this case, a target speed change ratio is preset according to driving conditions such as engine rotation speed and engine load. A control mechanism feedback controls the opening of the speed change ratio control valve so that, for example, the real rotational speed of the CVT output shaft coincides with a rotation speed corresponding to a set speed change ratio based on vehicle driving conditions.
There is thus a response delay until the real speed change ratio changes after the speed change ratio control valve opening is changed. Also, the oil pressure acting on each pulley of CVT is not necessarily directly proportional to the speed change ratio, and the dynamic characteristics of the speed change ratio when a change is made from one ratio to another are not uniform.
Accordingly, even when a real velocity is compared with a target velocity and the speed change ratio control valve is feedback controlled, a desired control response may not be obtained, the CVT output may give rise to hunting, and the vehicle and driver may experience a shock due to an excessive change of speed change ratio.